They were throwing fireworks into the openings in the horse trailer.

I'm starting to think this was supposed to be a false flag. On the day it happened, there were lots of false reports of horses being burned, but the last I read was that there were no horses in the trailer. There were also pictures posted of the driver putting a gun on the dashboard.

In any case, the woman who appeared to be injured ran from the front of the truck around to the side where she started her act.